1. Field of the Invention
The present invention relates to porous calcium zirconate/magnesia composites, and a method of producing the same, and in particular, relates to porous calcium zirconate/magnesia composites having a fine composite structure due to uniformly dispersed equimolar amounts of calcium zirconate [CaZrO3] and magnesia [MgO] and controlled grain growth, and a method of producing the same.
Porous CaZrO3/MgO composites with a uniform three-dimensional (3-D) network structure have been successfully synthesized using reactive sintering of highly-pure mixtures of natural dolomite, (CaMg(CO3)2), and synthesized zirconia powders with LiF additive. Equimolar dolomite and zirconia powders doped with 0.5 wt % LiF were cold isostatically pressed at 200 MPa and sintered at 1100-1400xc2x0 C. for 2 h in air. Through the liquid formation via LiF doping, strong necks were formed between constituent particles before completion of the pyrolysis of dolomite, resulting in the formation of a 3-D network structure. During and after the formation of network structure, CO2 was given off to form a homogeneous open-pore structure. The pore-size distribution was very narrow (with pore size: xcx9c1 xcexcm), and the porosity was controllable (e.g. xcx9c30-50%) by changing the sintering temperature. The porous composites can be applied as filter materials with good structural stability at high temperatures.
The porous composites of the present invention are useful as highly corrosion-resistant materials that function as fluid-permeable filters, lightweight members used at super-high temperatures, catalyst carriers, insulation, sound-absorbing materials, and the like.
2. Description of the Related Art
Porous ceramics having a high porosity made of oxides are used as conventional fluid-permeable filters, and the like. The conventional products are those in which pores are dispersed throughout a sintered compact by lowering the molding density and sintering temperature (controlled sintering), those in which pores are made by burning an organic binder (organic binder removal method), those in which uniform pores are made at a relatively low temperature using chemical means, such as alkoxide decomposition and reaction, etc. (sol gel method), and the like. Moreover, most of the porous products made by any of these production methods are single-phase porous compacts made from one ceramic.
However, these products have the following disadvantages: in conventional porous compact materials made by the controlled sintering method, bond strength between the crystal particles themselves that comprise the porous compact is insufficient and material strength of the porous compact overall is also insufficient, and because pore diameter distribution is wide, fluid selectivity is insufficient. There is a problem with porous compacts made by the organic binder removal method in that harmful gases such as NOx, and the like are generated corresponding to the components of the polymer when it is burned. Moreover, there is a problem in that it is difficult to control microstructure of the porous compact because heat is generated when the polymer is burned. In addition, although it is possible to control structure to a relatively high degree with porous compact materials made by chemical means, beginning with the sol gel method, high cost is required and therefore, there are problems in terms of mass production. Furthermore, there is a disadvantage with porous compact materials made by any of these production methods in that when they are used at high temperatures of 1,000xc2x0 C. or higher, sintering of the porous compact proceeds and structure thereof becomes coarser and pore diameter of the porous compact changes with time and as a result, properties thereof deteriorate.
Under these conditions, in light of the above-mentioned related art, the inventors have repeatedly performed intense research in order to develop new porous compact materials with which the above-mentioned problems can be solved, and successfully have completed the present invention upon developing porous calcium zirconate/magnesia composites having superior properties.
That is, the present invention provides porous calcium zirconate/magnesia composites whose structure is controlled to a high degree and which further have excellent heat resistance and corrosion resistance, and a method of producing these porous materials using a process that is advantageous in terms of cost in order to solve the above-mentioned disadvantages.
The present invention provides porous calcium zirconate/magnesia composites, and a method of producing the same.
The present invention relates to porous calcium zirconate/magnesia composites having a thermally and chemically stable porous structure, which consist of sintered compacts having a fine composite structure stable under high temperatures due to uniformly dispersed equimolar amounts of calcium zirconate [CaZrO3] and magnesia [MgO] and controlled grain growth, and also relates to a method of producing the same.
The present porous composites are useful as, for instance, highly corrosion resistant materials that function as fluid-permeable filters, lightweight members used at super-high temperatures, catalyst carriers, insulation, or sound-absorbing materials, and the like.
The present invention for solving the above-mentioned problems consists of the following technical means:
(1) Porous calcium zirconate/magnesia composites having a thermally and chemically stable porous structure,
said porous composites consist of sintered compacts having a fine composite structure stable under high temperatures due to uniformly dispersed equimolar amounts of calcium zirconate [CaZrO3] and magnesia [MgO] and controlled grain growth,
which are synthesized by using reactive sintering of equimolar mixture of dolomite and zirconia powders doped with liquid phase forming material.
(2) Porous calcium zirconate/magnesia composites according to above (1), wherein dolomite [CaMg(CO3)2] is used as the calcium source and magnesium source of the calcium zirconate and magnesia to achieve uniform mixing of the calcium and magnesium contained in the starting materials on an atomic level.
(3) A method of producing the porous calcium zirconate/magnesia composites defined in above (1) or (2), comprising:
uniformly crushing and mixing equimolar amounts of dolomite and zirconia powders doped with low-melting-point liquid phase forming material;
molding this mixture as needed; and
sintering the mixture to obtain the porous composites.
(4) A method of producing porous calcium zirconate/magnesia composites according to above (3), wherein natural dolomite ore of a high purity is used as the starting material and reacted with zirconia during the sintering to form an equimolar calcium zirconate/magnesia composite structure inexpensively and in a short time.
(5) A method of producing porous calcium zirconate/magnesia composites according to above (3), wherein the composites having a 3-dimensional network structure and high strength even though having a porosity of 40 to 60% are synthesized by using reactive sintering which comprises uniformly mixing equimolar amounts of dolomite and zirconia [ZrO2] with 0.5 to 2.0 wt % low-melting-point liquid phase forming material per the total amount of dolomite and zirconia, molding the mixture as needed, heating the mixture to promote intergranular dispersion via the formation of a liquid phase at a relatively low temperature (500 to 700xc2x0 C.), thereby forming strong necks between the dolomite and zirconia during the process of pyrolysis of the dolomite, liberating CO2 during the course of the subsequent rise in temperature, and sintering the mixture in atmosphere at 1,300 to 1,400xc2x0 C. to obtain the porous composites.
(6) A method of producing porous calcium zirconate/magnesia composites according to above (3), wherein an alkali fluoride selected from LiF or NaF is used as the low-melting-point liquid phase forming material.
The porous calcium zirconate/magnesia composites of the present invention are characterized in that they are sintered compacts having a fine composite structure with good stability at high temperatures due to uniformly dispersed equimolar amounts of both calcium zirconate [CaZrO3] and magnesia [MgO] and controlled grain growth.
The above-mentioned porous calcium zirconate/magnesia composites are produced by the process of uniformly mixing equimolar amounts of dolomite and zirconia [ZrO2] doped with 0.5 to 2.0 wt % low-melting-point liquid phase forming material per the total amount of dolomite and zirconia using natural dolomite ore of a high purity as the starting material, molding the mixture as needed, heating the mixture to promote intergranular dispersion via the formation of a liquid phase at a relatively low temperature (500 to 700xc2x0 C.) and thereby forming strong necks between the dolomite (or calcium carbonate and magnesium, as its primary decomposition products) and zirconia during the process of pyrolysis of the dolomite, liberating CO2 during the course of the subsequent rise in temperature, and sintering the mixture in air at 1,300 to 1,400xc2x0 C. to obtain the porous composites.
By using this process, it is possible to realize an equimolar calcium zirconate/magnesia composite structure inexpensively and in a short time and to realize a 3-dimensional network structure that is very strong even though it has a porosity of 40 to 60%. Moreover, since the LiF or NaF used as the low-melting-point liquid phase forming material is gasified through the open pores during the sintering process, there is a reduction in the amount that remains in the sintered compact and therefore, there are no detrimental effects on high-temperature properties.
Aspects of the present invention will be described in detail below.
The porous calcium zirconate/magnesia composites of the present invention have a fine composite structure due to uniformly dispersed equimolar amounts of both calcium zirconate and magnesia and controlled grain growth, and they retain a stable structure, even with long-term use under high temperatures, because grain growth of both the calcium zirconate and magnesia is controlled. Crystal particle diameter of calcium zirconate and magnesia does change somewhat depending on the sintering temperature, but because the composites have a 3-dimensional network structure, the mass transfer that is seen with bulk compacts rarely occurs and fine crystal particles of several microns or smaller are obtained. Almost all of the pores are homogeneous open through-pores and porosity is 40 to 60%, sufficient for selective permeation of fluids.
Individual calcium and magnesium sources are not used as the starting material. Dolomite (compound carbonate of calcium and magnesium) in which they are uniformly dispersed on a molecular level is used and therefore, there is extremely uniform dispersion of calcium and magnesium throughout the entire porous compact.
Moreover, the alkali fluoride added as the low-melting-point liquid phase forming material is released to outside the system through open pores and therefore, there is a reduction in the amount that remains in the porous compact and there is no deterioration of high-temperature properties. Necking of constituent ceramic particles occurs at a relatively low temperature because the low-melting-point liquid phase forming material has been added and therefore, mechanical strength is excellent and selective permeation of fluids with good efficiency is possible. Moreover, the porous compact of the present invention is extremely useful, even for other uses as a porous compact (for instance, lightweight members used at super-high temperatures, catalyst carriers, insulation, or sound-absorbing materials).
Ideally, natural dolomite of a high purity and synthetic zirconia of a high purity (or natural zirconia of a high purity is also possible) are used as the starting materials for the porous compact and LiF or NaF are used as the low-melting-point liquid phase forming material. It is of course possible to use synthetic dolomite as the starting material, but because it is possible to obtain inexpensive ore at a very high purity from specific places of origin, the use of natural dolomite of a high purity is ideal for realizing low cost. Commercial synthetic zirconia of high quality and high purity can be relatively inexpensively obtained and therefore, synthetic products are preferred. LiF or NaF is used as the low-melting-point liquid phase forming material, but the same results can be expected with fluorides of bivalent metals, such as CaF2, SrF2, BaF2, and the like. There will be a reduction in properties at high temperatures and there will also be detrimental effects on the sintering furnace if a large amount of low-melting-point liquid phase forming material remains in the sintered body and therefore, a small amount is preferred. It is possible to promote neck formation at a sufficiently low temperature (500 to 700xc2x0 C.) with a relatively small amount of 0.5 to 2.0% of the liquid phase forming material in the present invention.
Next, the method of producing the porous calcium zirconate/magnesia composites of the present invention will be described.
The porous composites of the present invention are produced by uniformly crushing and mixing equimolar amounts of dolomite and zirconia with 0.5 to 2.0 wt % low-melting-point liquid phase forming material additive per the total amount of dolomite and zirconia, and sintering the mixture in air. Pyrolysis of the dolomite, neck formation by the low-melting-point liquid phase forming material, formation of calcium zirconate by dominant reaction between the calcia [CaO], which is a decomposition product of dolomite, and zirconia, and evaporation of the low-melting-point liquid phase forming material occur during this sintering process. The method of the present invention is particularly effective in lowering cost because this type of complex process can be realized by simple heat treatment in just one step.
The dolomite can be coarser particles than the zirconia because a fine decomposition product nanometers in size is made during pyrolysis. However, fragmentation of the dolomite is an effective way of realizing a uniform reaction with the zirconia and the use of a high-energy process, such as a planetary ball-mill, vibrating ball-mill, attriting ball-mill, and the like are preferred as the mixing and crushing method.
Sintering can be performed in an ordinary furnace with an air atmosphere. It is also possible to recover the gas that is generated by setting up an appropriate CO2 trap so that there are no detrimental effects on the environment.
The porous calcium zirconate/magnesia composites of the present invention that are obtained in this way have a fine composite structure that remains stable under high temperatures due to uniformly dispersed equimolar amounts of both calcium zirconate and magnesia and controlled grain growth, and therefore, the above-mentioned improved properties of porous compacts are realized.